[SQL] 리트코드 183. Customers Who Never Order

문제

Table: Customers

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| id          | int     |
| name        | varchar |
+-------------+---------+

id is the primary key (column with unique values) for this table.
Each row of this table indicates the ID and name of a customer.

 

Table: Orders

+-------------+------+
| Column Name | Type |
+-------------+------+
| id          | int  |
| customerId  | int  |
+-------------+------+

id is the primary key (column with unique values) for this table.
customerId is a foreign key (reference columns) of the ID from the Customers table.
Each row of this table indicates the ID of an order and the ID of the customer who ordered it.

 

Write a solution to find all customers who never order anything.
Return the result table in any order.
The result format is in the following example.

 

https://leetcode.com/problems/customers-who-never-order/description/

Customers Who Never Order - LeetCode

예시

Input: 
Customers table:

+----+-------+
| id | name  |
+----+-------+
| 1  | Joe   |
| 2  | Henry |
| 3  | Sam   |
| 4  | Max   |
+----+-------+

Orders table:

+----+------------+
| id | customerId |
+----+------------+
| 1  | 3          |
| 2  | 1          |
+----+------------+

Output: 

+-----------+
| Customers |
+-----------+
| Henry     |
| Max       |
+-----------+

문제 풀이

SELECT Customers.name AS Customers
FROM Customers			--Customers table을 기준으로 LEFT JOIN
LEFT JOIN Orders ON Customers.ID = Orders.customerID
WHERE Orders.id IS NULL --주문 이력이 없는 고객만 출력

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SQL을 독학하시는 분들에게 도움이 되길 바라며,

혹 더 좋은 방법이 있거나 오류가 있다면 편하게 말씀 부탁드립니다.